The present invention relates to annular cutters for use in cutting holes in a workpiece, preferably a metal workpiece. More particularly, the present invention relates to an improved annular cutter which cuts holes more efficiently than previously known cutters because of its ability to discharge chips more effectively. As is known, annular cutters cut a hole in a workpiece by cutting an annular groove or kerf into the workpiece with an axial central slug of material remaining after the hole is cut. The cutting teeth of the annular cutter continually remove material from the bottom of this kerf in the form of chips which are discharged through flutes formed in the body of the annular cutter, generally in the exterior of the body.
Experience has shown that the life and the efficiency of an annular cutter (i.e. the ease and the number of times in which it can cut holes in metal workpieces and the finish produced by the annular cutter in the metal workpiece) are to a very large extent dependent upon the ability of the cutter to discharge the cut material or chips through its flutes or discharge passages. When the chips formed by an annular hole cutter cannot move freely away from the cutting edges or the flutes become packed or clogged with chips, the torque and thrust required to feed the cutter increases, the cutter wears more rapidly and the finish of the hole deteriorates. This is due primarily to the chips congesting between the cutting edges and the workpiece, creating excessive frictional heat build-up which results in cutting edge abrasion and damage to the sidewall of the hole being cut.
The applicant of the present invention recognized the need for free chip discharge and has invented several patented annular cutters which discharge chips more freely.
In applicant's U.S. Pat. No. 3,609,056 ('056), an annular cutter is disclosed wherein each tooth cuts a single chip. The successive teeth are divided into groups of three with each tooth in each group cutting a chip having a width of about one-third the width of the tooth. A thickened portion 18 is provided to facilitate the discharge of chips by providing a clearance area 104. This clearance area 104 provides an escape passage which becomes progressively wider in an outer direction.
The '056 cutter has been successful in improving efficiency and cutter life by improving chip discharge, but the design sacrifices speed because each tooth cuts only one-third of the kerf as compared to an annular cutter having each tooth cutting the entire kerf.
Recognizing this disadvantage, Applicant invented the annular cutter of U.S. Reissue Pat. No. Re 28,416 ('416). The '416 patent discloses an annular cutter wherein each tooth is formed with at least two radially-extending, circumferentially-staggered cutting edges. The cutting edges are designed so that each cutting cuts a chip from the kerf with a slight overlap of the cutting edges so that the entire kerf is cut by each tooth. As will be understood, this cutter will cut a hole faster than the '056 patent if the same number of teeth are used on both cutters.
The radially innermost cutting edge of the cutter disclosed in the '416 patent extends radially across the shallow gullet formed in the web between successive teeth, and the outermost cutting edge extends radially across the trailing wall of the flute which extends spirally upwardly of the cutter between successive teeth. Both the flute and the web have a thickness equal to about one-half the thickness of the annular wall of the cutter. These two cutting edges are separated circumferentially by the bottom wall of the flute which, in the embodiment disclosed in the '416 patent, is inclined slightly in a radially inward direction so that the cutting edges overlap radially a slight extent to cut two separate chips or at least a chip which fractures easily into two chips. This slight incline is illustrated in FIG. 4 of the '416 patent. The incline begins at 38 and extends back to 39. As can be seen, the end 48 of cutting edge 22 slightly overlaps the end 39 of cutting edge 24.
The cutter disclosed in the '416 patent produces cutting actions which are far superior to cutters previously used and to this day is extremely successful. However, it was found that when the cutter described in the '416 patent was used to cut holes on a production basis, high speeds and heavy usage, there is a tendency for the chips to clog and not move as freely as desired. This causes the cutting action to be much slower, and tapered, oversized holes are produced with a coarse finish. Further, the life of the cutter is shortened, and the cutter may be broken in heavy use.
It was then determined by applicant that the most practical way of overcoming the disadvantages of '416 patent, while maintaining its advantages, was to design a cutter which would produce thin, narrow chips that could be easily directed into the flute as soon they were cut. It was believed by the applicant that a wide chip would not bend readily and would occupy a relatively large volume; therefore, if a chip was narrow, it would encounter less obstruction in being discharged. Further, it was known that as the size of the flute is reduced, the strength of the cutter is increased since the web between the successive teeth is thicker.
To obtain the advantages of a thinner chip and a stronger cutter, applicant invented the cutter described in U.S. Pat. No. 4,452,554 ('554) which has a plurality of at least three cutting edges on each tooth. Each of these cutting edges has a radial dimension substantially less than one-half the wall thickness and preferably equal to about one-third the wall thickness. In this way, the radial dimension of the flute could be as small as about one-third the wall thickness and still sufficiently deep to freely accommodate the chip cut by the widest cutting edge.
The annular hole cutter disclosed in applicant's '554 patent has proven to be a commercial success having substantial advantages over previously known cutters, including improved efficiency and tool life. However, it was discovered there was still a problem with clogging at high speeds and heavy usage encountered in certain industrial applications. It was discovered that the problem was due to the chip that was cut by the outer cutting edge. When the outer cutting edge cuts a chip, the chip may become wedged between the inner wall of the hole being formed and the shoulder defined between the stepped teeth. In cutters having circumferentially staggered cutting edges, the outer cutting edge terminates at its radially inner end against the circumferentially extending shoulder of the cutter and terminates at its radially outer end at the inner wall of the hole being formed. If the chip cut by this outer cutting edge is not narrower than this chip passage are defined between the shoulder and inner wall, the chip may become wedged.
It was a limitation of both the '416 and '554 patents that regardless of what changes were made to the flute depth to accommodate the inner chip, the chip cut by the outer edge was wedging between the shoulder and the inner wall of the hole.
After extensive experimentation and investigation, it was discovered by applicant that this interference at the outer chip resulted primarily from three factors working separately or in combination, namely: (1) the generated chip width is greater than the chip passage or flute depth adjacent the cutting edge, (2) chip expansion after the chip is cut and (3) radial movement of the chips as they are cut, this movement being primarily radially inward movement.
The total or actual width of the cutting edge of the tooth will be equal to the kerf width of that edge, i.e. the path being cut by that edge, only when that edge is straight and perpendicular to the axis of rotation of the annular cutter. Any other form of cutting edge, such as straight but not perpendicular to the axis of rotation or crested, etc. will have a total width greater than the kerf width cutting a chip having a generated width which is greater than the kerf width. Typically, larger generated chip widths will occur in cutters having crests. With reference to the '554 patent, the outside cutting edge 38 has a crest 60 which is defined by the intersection of the back-off faces. Cutting edge 38 will cut a chip which is initially bent, but which will flatten immediately after it is cut. Previously, it was believed that these chips did not flatten, but rather the chips folded so that they could easily flow between the passage defined by shoulder 28 and the inner wall of the hole being cut.
Chip expansion is believed by the applicant to be due to linear deformation of the chip with resultant lateral or radial expansion of the chip. This chip expansion is believed by applicant to be dependent upon the feed rate and the geometry of the teeth, wherein a greater feed rate results in a thicker chip which expands radially more than a thinner chip. It has been found in testing that a tooth having a flat cutting edge defined by an acute angle of 15 degrees and kerf width of 0.140 inches, for example, will cut a chip having a radial width of 0.152 inches for mild low-carbon steel at a feed rate of about 0.007 inches per tooth. This is 0.012 inches wider than the kerf width. In a cutter having crested teeth, the chip expansion is somewhat less; however, the generated width of the chip, due to the crested teeth, will still give a substantially wider chip than the kerf width.
Finally, it has been found in testing that chips are normally generated inwardly with an annular hole cutter, toward the axis of the cutter, resulting in further interference.
The above problems were resolved in U.S. Pat. No. 4,632,610 ('610) issued to the inventor of the present application. In the '610 patent, portions of the cutting edges of pairs of teeth were alternately relieved in such a way that the kerf was further subdivided. The outer chip, for the first time, was narrower than the adjacent chip passage. While this greatly enhanced the efficiency and cutting ability of the tool, especially in heavier feeds and deeper holes, it complicated the construction of the tool and made it more difficult to manufacture and resharpen. Although the cutter disclosed in the '610 patent has proven to be a commercial success, the applicant continued to seek a simpler solution to this very difficult problem.
The present invention solves the problem of chip wedging at the outer tooth, while maintaining a design that is simple to manufacture and resharpen. Further, the present invention may be utilized on any annular hole cutter having adjacent teeth which are circumferentially spaced so that a shoulder is defined between the cutting edges, and the cutting edges form two separate chips.